Solder joints between lead wires and terminal conductor pads or between two wires may temporarily provide good electrical conduction but lack the mechanical strength to survive physical shock and fatigue in their intended use. For example, discrete elements on printed circuit boards may initially have a continuous electrical contact through a solder joint to the printed conductor, but when used in mobile systems may be subject to vibration causing cracks or physical discontinuities in the solder joint resulting in a current interruption. Therefore, electrical, quality-control testing would be insufficient to detect physical defects in the solder joint which may cause the joint to fail in use. Such defects might be related to poor adherence of the solder and wire, inclusions, porosity, cold solder, a break or other similar flaws.
Commonly available ultrasonic flaw detectors apply an acoustic wave to a material and monitor and receive an echo from the flaw (usually an inclusion or crack).